1. Field of the Invention
The present invention generally relates to a radio telephone and a method for selecting a radio channel. More specifically, the present invention is directed to a radio telephone system in which a portable radio telephone of a private-use radio telephone apparatus connected to a telephone communication network, can communicate also with a public-use radio base station connected to the communication network.
2. Description of the Prior Art
The above-described private-use radio telephone apparatus is known as a so-called "cordless telephone" used in a home, and also a so-termed "system cordless telephone" utilized in a firm. This radio telephone system has been developed in Japan as a second-generation cordless telephone system. In the second- generation cordless telephone system, a plurality of public radio base stations and a control center for controlling communications via these public radio base stations, are connected to the presently available communication network, namely, a PSTN (Public Switched Telephone Network) or an ISDN (Integrated Services Digital Network). To this communication network, cordless telephones used in homes and offices are connected. The portable telephone terminals (radio telephones) of these cordless telephones are registered in the control center in order that these portable telephone terminals can also be operated outside the homes and offices. In other words, a portable telephone terminal of a cordless telephone apparatus is operable as a handset of this cordless telephone apparatus while this portable telephone terminal is used within a range covered by a home, or a firm, and furthermore, the portable telephone terminal is operable like a "cellular phone" while this portable telephone terminal is brought out from the home or firm in the second-generation cordless telephone system.
Also in this second generation cordless telephone system, a radio telephone is designed to intermittently receive control channel information in synchronism with a transmission period of a radio base station in a similar manner to that of the normal portable radio telephone system. This control channel information is similarly, intermittently transmitted from the radio base station, and contains such information to require a telephone call.
Under such a circumstance, first of all, one receiving method for a portable terminal of a cordless telephone apparatus which has been registered into the control center may be conceived such that both of control channel information with regard to the radio base station of the cordless telephone apparatus (simply referred to a "private radio base station" hereinafter) and the public radio base station are received at the same time. Even if the portable terminal would be designed to be receivable both of the control channel information for the different radio base stations, while this portable terminal is brought out from a home or office, the control channel information of the private radio base station can be hardly received and useless power thereof is consumed.
As a consequence, it is preferable for the portable terminal receiving method to receive the control channel information from the private radio base station when the portable terminal is operated within the home or office, and also to receive the control channel information of the public radio base station when the portable terminal is brought out from the home or office. One possible and simple receiving method is conceived to achieve the above-described preferable conditions. That is, a selecting switch may be employed and manipulated to receive only one control channel information.
However, in this case, the following drawback may be caused under such an environment that both of the control channel information transmitted from the private radio base station and the public radio base station can be received, namely if the selecting switch provided in the portable terminal would be selected to receive the signal derived from the public radio base station and this portable terminal would be operated near the home, or office. That is, when a telephone call is made, although this telephone call could be made by the portable terminal via the private radio base station at a free communication cost, this telephone call is made via the public radio base station and therefore requires a certain amount of communication fee. As a result, a total communication fee becomes expensive.